1. Field of the Invention
The invention relates to pulverulent amorphous solids having a very small median particle size and a narrow particle size distribution, a process for the preparation thereof and the use thereof.
2. Discussion of the Background
Finely divided, amorphous silica and silicates have been produced industrially for decades. As a rule, the very fine milling is carried out in spiral jet mills or opposed jet mills using compressed air as milling gas, e.g. EP 0139279.
It is known that the achievable particle diameter is proportional to the square root of the inverse of the impact velocity of the particles. The impact velocity in turn is predetermined by the jet velocity of the expanding gas jets of the respective milling medium from the nozzles used. For this reason, superheated steam can preferably be used for generating very small particle sizes, since the acceleration power of steam is about 50% greater than that of air. However, the use of steam has the disadvantage that condensation may occur in the entire milling system, particularly during the startup of the mill, which as a rule results in the formation of agglomerates and crusts during the milling process.
The median particle diameters d50 achieved with the use of conventional jet mills in the milling of amorphous silica, silicates or silica gels have therefore been substantially above 1 μm to date. Thus, for example, U.S. Pat. No. 3,367,742 describes a process for milling aerogels, in which aerogels having a median particle diameter of 1.8 to 2.2 μm are obtained. Milling to a median particle diameter of less than 1 μm is, however, not possible with this technique. Furthermore, the particles of U.S. Pat. No. 3,367,742 have a broad particle size distribution with particle diameters of 0.1 to 5.5 μm and a fraction of 15 to 20% of particles >2 μm. A large fraction of large particles, i.e. >2 μm, is disadvantageous for applications in coating systems since as a result thin coats having a smooth surface cannot be produced. U.S. Pat. No. 2,856,268 describes the combined milling and drying of silica gels in vapour jet mills. However, the median particle diameters achieved thereby were substantially above 2 μm.
An alternative possibility for milling is wet comminution, e.g. in ball mills. This leads to very finely divided suspensions of the products to be milled, cf. for example WO 200002814. It is not possible with the aid of this technology to isolate a finely divided, agglomerate-free dry product from these suspensions, in particular without changing the porosymmetric properties.